The only way to flash the image in TL-MR3020 v3 is to use
tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.0.225/24 and tftp server.
2. Rename "openwrt-ramips-mt76x8-tplink_tl-mr3020-v3-squashfs-tftp-recovery.bin"
to "tp_recovery.bin" and place it in tftp server directory.
3. Connect PC with the LAN port, press the reset button, power up
the router and keep button pressed for around 6-7 seconds, until
device starts downloading the file.
4. Router will download file from server, write it to flash and reboot.